This invention relates to a film-transport mechanism. More particularly this invention concerns such a mechanism which drives a perforated strip film, such as a motion-picture film, in either a reverse or forward direction in a projector.
An intermittent film advance mechanism is known such as described in my earlier commonly assigned U.S. Pat. No. 3,860,332, the entire disclosure of which is herewith incorporated by reference. In this arrangement a claw carried on a film-transport element pivoted on the projector housing is moved by means of a cam so that the claw is first inserted into a perforation in the film, then pushes the film longitudinally through an increment, then withdraws from this perforation, and finally returns to its original position so as to be aligned with the immediately trailing perforation of the film so that it can again engage therein and advance the film another step or increment. During the actual displacement of the film, which is as mentioned above intermittent, a rotary disk shutter temporarily prevents light from the lamp and condenser of the projector from shining through the film onto the screen.
The cam that operates the film-transport element is typically of the rotary type having an annular cam surface surrounding the rotation axis of the cam. The radially inner or outer surface of this cam, depending on its type, is contoured for a gentle acceleration of the film so that this film is brought up to transport speed gently. In addition an axial face of this cam is contoured in order to raise and lower the claw from the film, thereby withdrawing and inserting it in the perforation thereof.
Such an arrangement has proven itself highly efficient in the forward advance of a film. The particularly gentle action of the feed claw prevents damage to the film and insures that each frame of the film is exactly aligned with the lens and disk shutter for best projection results. Nonetheless this system has the disadvantage that it cannot be readily reverse-driven during projection of the film so as to allow the film to be projected while being reverse-driven.
The main problem during such reverse operation is that the section of the cam responsible for the film transport itself is so shaped as to bring the claw gently into contact with the leading edge of the perforation in the film, and thereafter to move the claw with ever increasing speed to displace the film smoothly from a complete stop. Once the film has been advanced into the desired position it can be stopped relatively abruptly due to its small mass so that the claw need merely be pulled out of the perforation. When operated in reverse this means that the claw will be brought rather sharply into contact with the edge of the perforation, thereby generating considerable noise and simultaneously damaging the film or at least causing it to wear at a much greater rate.
Another disadvantage of this arrangement is that the claw is somewhat smaller than the perforation, so that when reverse-driven the frames of the film will not be exactly aligned with the projector lens. Thus readjustment of the projector is necessary for such reverse-driving.
Another disadvantage of these systems is that when driven in reverse the frames of the film are stationary during a different time related to the shutter plate than when driven forward. Thus it is necessary to modify the disc shutter so that it blocks out the film for a longer period of time. This decreases overall image brightness and requires yet another modification of the projector.